Table 1 defines prior nickel based compositions suitable for use in rotor discs for gas turbine engines, such as high pressure compressor and turbine discs. These include the following compositions described in the corresponding documents: Udimet™ 720Li (described in U.S. Pat. No. 4,093,476); RR1000™ (described in U.S. Pat. No. 6,132,527); ME3, also known as René 104™ (described in U.S. Pat. No. 6,521,175); LSHR (described in U.S. Pat. No. 6,974,508); Alloy 10 (described in U.S. Pat. No. 6,468,368); Maurer et al (described in U.S. Pat. No. 4,629,521) and Allvac 718 Plus™ (described in U.S. Pat. No. 6,730,264).
Nickel based alloys for use in gas turbine engine components such as high pressure rotor discs have a number of requirements. They must be resistant to environmental degradation such as hot corrosion and oxidation, have a high yield strength at high temperatures, be resistant to creep strain accumulation and dwell fatigue, have a low density, and a good surface stability. In the art, the “stability” of an alloy is normally understood to refer to the alloy's propensity to precipitate detrimental phases (i.e. an alloy having a high stability will have a low propensity to precipitate detrimental phases). An example of a detrimental phase is the sigma (σ) phase, which can occur when the alloy is subjected to high temperatures for extended time periods (known as “dwell”). One aim of the invention is therefore to minimise the volume fraction of the σ phase after a given time at a given anticipated operating temperature (generally between around 700° C. and 800° C.).
It is also desirable for the alloy composition to have a low cost, and be suitable for low cost production methods such as forging and powder metallurgy. Alloys having the above properties, and therefore being suitable for use in gas turbine engine components such as turbine blades and rotors discs, are generally known in the art as “superalloys”, and are sometimes also referred to as “high performance alloys”.
Of these above requirements, the yield strength of the composition at high temperatures (i.e. between around 700 to 800° C.) is considered to be most important. The prior nickel based alloys listed in Table 1 have an operational temperature range between 700 and 750° C. Above this temperature, the yield strength of these alloys has been found to be insufficient for some applications, such as for rotor discs in gas turbine engines.
Increasingly, nickel based alloys used for disc rotor applications in modern gas turbine engines for civil aircraft are expected to operate for longer periods of time at temperatures above 700° C., and in some cases up to 800° C. Rotor discs capable of operating at these temperatures are desirable in order to increase compressor discharge temperatures and rotational speeds, particularly for high pressure compressor and turbine rotors, as both of these factors will lead to increased turbine inlet temperatures, and therefore improved overall thermal and fuel efficiency of the gas turbine engine employing these discs.
FIG. 2 shows a cross sectional side view of part of a typical rotor disc 40 having an attached blade 41. The disc 40 comprises a radially outer rim 42, and diaphragm 44 extending from an inner annulus 45 (also known as a bore) of the disc 40 to the rim 42.
Both the rim 42 and diaphragm 44 of disc rotors 40 can be exposed in use to dwell or sustained load-fatigue cycles at these high temperatures. Under these conditions, stress-assisted oxidation and time dependent deformation can produce intergranular crack growth and therefore high rates of crack growth in discs made of prior alloys. Similar discs 40 are used for both the compressor and turbine sections of the engine. As a result, resistance to dwell crack growth, and damage tolerance, is a priority for new superalloys for turbine discs. Secondly, many of the prior alloys described in Table 1 have compromised resistance to surface degradation caused by environmental conditions (such as oxidation and type II hot corrosion damage) for improved high temperature strength, resistance to creep strain accumulation and for stable bulk material microstructures (to prevent the precipitation of detrimental topological close packed (TCP) phases). As discs are being exposed to increasingly high temperatures, exceeding 700° C., oxidation and hot corrosion damage has become a significant limiting factor for component life. There is now a need to prioritise resistance to hot corrosion and oxidation damage, ahead of other properties when defining alloy chemistries, while maintaining sufficient yield strength at these high temperatures, at an acceptable cost. If prior alloys are used at such high temperatures, environmental protection will need to be applied to disc rotors (e.g. surface coatings), which is technically very difficult and undesirable due to the increased weight and complexity of such discs and the additional manufacturing steps required to form such discs.
The present invention seeks to provide an improved alloy composition, an improved rotor disc, an improved gas turbine engine, and an improved method of forming an article which solves some or all of the above problems.